A Study Of The Cambrian Furongian Paleo Marine Environment In North China

Cambrian is an important period for the frequent occurrence of life eruption and extinction,along with the changes of paleoclimate and paleooceanic chemical conditions.These variations can be reflected in the long-term changes in the sedimentary characteristics of Cambrian strata,isotopes and elemental geochemistry.The occurrence of global extinction event in the early stage of the Late Cambrian?Furongian?is consistent with the carbon isotope excursion.However,there is still considerable confusion on explanation of this correlation and understanding of some issues about the changes.The point is that there is still some lack of knowledge about relationships between the changes of paleo marine environment and biological evolutions during the Furongian.Typical sections in North China were selected to assess the chemical evolution of the late Cambrian seawater and the characteristics of biogeochemical cycle.Global comparison involving North China,South China and other paleoplates allows interpretation of the processes and origin.The main conclusions and understandings include the following:Field investigation and microscopic petrographic observation reveal that the Upper Cambrian is composed of open platform carbonates.A history of sea-level fluctuations during this period is reconstructed by using stratigraphic sections and geochemical data:The sea level shows a general trend of decline-rise-decline from the Gushan period to the Fengshan period,with a short-lived but strong fluctuation?rapid rise and fall?in the transition period from the Gushan period to the Changshan period.Geochemical sections were constructed for the Upper Cambrian strata in North China.The data indicate that both the temperature and salinity of the paleoseawater show a gradual rise through the Late Cambrian.During the Furongian,the seawater was characterized by reducing,enriched nutrient elements and moderate contents of toxic elements.The carbon isotopic sections show that a positive shift of carbon isotopes is present in the Furongian Changshan Formation in North China.The carbon isotope excursion can be attributed to the rapid burial of numerous ¹²C-rich organic matter in deep water and the resulting enrichment of ¹³C in the sea water.The intensification of weathering related to the sea level drop resulted in input of abundant nutrients into the seawater,which promoted the proliferation of algae and phytoplankton.The combined occurrence of the abrupt drop of seawater temperature,the elevation of salinity and the input of toxic elements led to the trilobite extinction.The amplitude of the carbon isotope excursions in North China is significantly lower than that in South China and other regions around the world.The reason is that the sedimentary environment of the study area is open platform facies,and the paleohighs alone the platform margin would have acted as barrier for expansion of ¹³C-rich anaerobic seawater from basin facies to platform facies.Meanwhile,the development of the broad platform had probably placed certain constraints on the input and distribution of the ¹²C rich terrigenous water,which might have exerted profound influence on the carbon isotopes of the shallow water bodies rather than on those of the deep water bodies on the slope facies and basin facies.This explains the presence of the worldwide trend of the carbon isotopes increasing from the shallow-water carbonates to deep-water carbonates.